# Physics problem - electrostatic unit?

I've tried solving this question with three different methods but I'm stuck with which one is correct... Please help!

Here is the question, I attached a diagram below:

A 30cm string has a 5 gram pith ball attached to each end. The string is hung over a meter stick and due to an electrostatic repulsion, the string makes an angle of 28 degrees where it is hanging. Assuming the charge on each pith ball is the same, determine the charge of the pith ball.

### 1 Answer

- NCSLv 73 months agoFavorite Answer
half the angle is Θ = 14º

and so the distance between pith balls is

d = 2*15cm*sin14º = 7.26 cm

The geometry of the string gives the geometry of the forces:

tan14º = Fe / m*g = Fe / 0.005kg*9.8m/s²

electrostatic force Fe = 0.01222 N

But Fe = kQq / d²

0.01222 N = 8.99e9N·m²/C² * q² / (0.0726m)²

q = 8.46e-8 C ≈ 85 nC

both negative or both positive

Hope this helps!